Cluster spin glass behavior in geometrically frustrated Zn3V3O8

High-Temperature Ferroic Glassy States in SrTiO_{3}-Based Thin Films

Disordered ferroics hold great promise for next-generation magnetoelectric devices because their lack of symmetry constraints implies negligible hysteresis with low energy costs. However, the transition temperature and the magnitude of polarization and magnetization are still too low to meet application requirements. Here, taking the prototype perovskite of SrTiO_{3} as an instance, we realize a coexisting spin and dipole reentrant glass states in SrTiO_{3} homoepitaxial films via manipulation of local symmetry. Room-temperature saturation magnetization and spontaneous polarization reach ∼ 10  emu/cm^{3} and ∼ 25  μC/cm^{2}, respectively, with high transition temperatures (101 K and 236 K for spin and dipole glass temperatures and 556 K and 1100 K for Curie temperatures, respectively). Our atomic-scale investigation points out an underlying mechanism, where the Ti/O-defective unit cells break the local translational and orbital symmetry to drive the formation of unusual slush states. This study advances our understanding of the nature of the intricate couplings of ferroic glasses. Our approach could be applied to numerous perovskite oxides for the simultaneous control of the local magnetic and polar orderings and for the exploration of the underlying physics.

Emergence of low-temperature glassy dynamics in Ru substituted non-magnetic insulator CaHfO3

Non-magnetic insulators/semiconductors with induced magnetism introduced via transition metal substitution are one of the promising materials in the field of spintronics, magnetoelectronics and magneto-optical devices. In this context, here, we focus on magnetism induced in a non-magnetic insulator CaHfO₃, by the substitution of 4d element Ru, at Hf-site. Structural investigations indicate that substitution of Ru⁴⁺(up to 50%) does not affect the original crystal structure of the parent compound. Magnetic studies divulge a crossover from a diamagnetic to paramagnetic state with 20% Ru substitution. Further replacement of Hf results in a glassy magnetic state in CaHf_1-xRu_xO₃(0.3 ⩽x⩽ 0.5). The nature of the low temperature glassiness (below 20 K) in these compositions is confirmed through Vogel-Fulcher and Power law, along with, magnetic memory effect and relaxation dynamics. The observed glassiness is explained through the phenomenological 'hierarchical model'. Our studies indicate that the presence of competing short range interactions among randomly arranged Ru cations in non-magnetic insulator CaHfO₃are responsible for the observed low temperature magnetic state in this series with compositions >0.25.

Effects of Non-Stoichiometry on the Ground State of the Frustrated System Li0.8Ni0.6Sb0.4O2

The non-stoichiometric system Li0.8Ni0.6Sb0.4O2 is a Li-deficient derivative of the zigzag honeycomb antiferromagnet Li3Ni2SbO6. Structural and magnetic properties of Li0.8Ni0.6Sb0.4O2 were studied by means of X-ray diffraction, magnetic susceptibility, specific heat, and nuclear magnetic resonance measurements. Powder X-ray diffraction data shows the formation of a new phase, which is Sb-enriched and Li-deficient with respect to the structurally honeycomb-ordered Li3Ni2SbO6. This structural modification manifests in a drastic change of the magnetic properties in comparison to the stoichiometric partner. Bulk static (dc) magnetic susceptibility measurements show an overall antiferromagnetic interaction (Θ = -4 K) between Ni2+ spins (S = 1), while dynamic (ac) susceptibility reveals a transition into a spin glass state at a freezing temperature TSG ~ 8 K. These results were supported by the absence of the λ-anomaly in the specific heat Cp(T) down to 2 K. Moreover, combination of the bulk static susceptibility, heat capacity and 7Li NMR studies indicates a complicated temperature transformation of the magnetic system. We observe a development of a cluster spin glass, where the Ising-like Ni2+ magnetic moments demonstrate a 2D correlated slow short-range dynamics already at 12 K, whereas the formation of 3D short range static ordered clusters occurs far below the spin-glass freezing temperature at T ~ 4 K as it can be seen from the 7Li NMR spectrum.

Spinel Zn3V3O8 nanosheets via a one-step hydrothermal synthesis with peroxidase-like activity for high sensitivity glucose colorimetric detection in synthetic perspiration

Due to their specific spinel structure, ternary oxides with multi-catalytic sites on a highly active exposed surface are recommended as alternative bio-catalysts. Spinel zinc vanadate with two-dimensional nanosheets (Zn3V3O8 NSs) was synthesised using a one-step hydrothermal route with CTAB and glycine as a bi-surfactant, where each NS has a thin thickness (25 nm) and wide cross section (2 μm). As a key parameter for peroxidase-like activity, the Michaelis-Menten constant (Km) for Zn3V3O8 NSs was calculated to be 0.271 mM with TMB and 1.317 mM with H2O2 at optimum conditions, indicating a higher affinity for the exposed (011) facet towards horseradish peroxidases. This affinity is related to the geometric matching between V4+ active sites and the terminal amino groups of TMB. The V4+ ions on the (011) facet act as dangling bonds and readily react with H2O2 in a Fenton-like reaction. The peroxidase-like activity for Zn3V3O8 NSs is verified by the formation of [V(IV)-OO˙] by the ˙O2- and V5+ near V4+ sites, but oxidase activity for Zn3V3O8 NSs. Based on the peroxidase-like activity, Zn3V3O8 NSs were used as a colorimetric glucose sensor with a wide linear range from 0.01 to 0.5 mM and a detection limit (LOD = 3σ/S) of 2.81 × 10-7 M. The colorimetric sensor also exhibited high accuracy and selectivity in synthetic perspiration samples.

Cluster Frustration in the Breathing Pyrochlore Magnet LiGaCr_{4}S_{8}

We present a comprehensive neutron scattering study of the breathing pyrochlore magnet LiGaCr_{4}S_{8}. We observe an unconventional magnetic excitation spectrum with a separation of high- and low-energy spin dynamics in the correlated paramagnetic regime above a spin-freezing transition at 12(2) K. By fitting to magnetic diffuse-scattering data, we parametrize the spin Hamiltonian. We find that interactions are ferromagnetic within the large and small tetrahedra of the breathing pyrochlore lattice, but antiferromagnetic further-neighbor interactions are also essential to explain our data, in qualitative agreement with density-functional-theory predictions [Ghosh et al., npj Quantum Mater. 4, 63 (2019)2397-464810.1038/s41535-019-0202-z]. We explain the origin of geometrical frustration in LiGaCr_{4}S_{8} in terms of net antiferromagnetic coupling between emergent tetrahedral spin clusters that occupy a face-centered-cubic lattice. Our results provide insight into the emergence of frustration in the presence of strong further-neighbor couplings, and a blueprint for the determination of magnetic interactions in classical spin liquids.

Lattice strain accommodation and absence of pre-transition phases in Ni50Mn25+xIn25-x

The stoichiometric Ni₅₀Mn₂₅In₂₅Heusler alloy transforms from a stable ferromagnetic austenitic ground state to an incommensurate modulated martensitic ground state with a progressive replacement of In with Mn without any pre-transition phases. The absence of pre-transition phases like strain glass in Ni₅₀Mn_25+xIn_25-xalloys is explained to be the ability of the ferromagnetic cubic structure to accommodate the lattice strain caused by atomic size differences of In and Mn atoms. Beyond the critical value ofx= 8.75, the alloys undergo martensitic transformation despite the formation of ferromagnetic and antiferromagnetic clusters and the appearance of a super spin glass state.

Emergent Spin Glass Behavior Created by Self-Assembled Antiferromagnetic NiO Columns in Ferrimagnetic NiFe2O4

Spin glass (SG) is a magnetic state with spin structure incommensurate with lattice and charge. Fundamental understanding of its behavior has a profound impact on many technological problems. Here, we present a novel case of interface-induced spin glass behavior via self-assembly of single-crystalline NiO microcolumns in a single-crystalline NiFe₂O₄ matrix. Scanning transmission electron microscopy indicates that the hexagonal-shaped NiO columns are along their [211] direction and oriented along the [111] direction of the NiFe₂O₄ matrix. Magnetic force microscopy reveals magnetic anisotropy between NiO columns (antiferromagnetic transition temperature T_N ∼ 523 K) and NiFe₂O₄ matrix (ferrimagnetic transition temperature T_FI ∼ 860 K). This leads to spin disorder/frustration at atomically sharp NiFe₂O₄/NiO interfaces responsible for spin glass behavior below T_SG ∼ 28 K. Our results demonstrate that self-assembly of magnetically distinct microstructures into another crystalline and magnetically ordered matrix is an effective way to create novel spin states at interfaces.

Study of Polycrystalline Bulk Sr3OsO6 Double-Perovskite Insulator: Comparison with 1000 K Ferromagnetic Epitaxial Films

Polycrystalline Sr₃OsO₆, which is an ordered double-perovskite insulator, is synthesized via solid-state reaction under high-temperature and high-pressure conditions of 1200 °C and 6 GPa. The synthesis enables us to conduct a comparative study of the bulk form of Sr₃OsO₆ toward revealing the driving mechanism of 1000 K ferromagnetism, which has recently been discovered for epitaxially grown Sr₃OsO₆ films. Unlike the film, the bulk is dominated by antiferromagnetism rather than ferromagnetism. Therefore, robust ferromagnetic order appears only when Sr₃OsO₆ is under the influence of interfaces. A specific heat capacity of 39.6(9) × 10^-3 J mol^-1 K^-2 is found at low temperatures (<17 K). This value is remarkably high, suggesting the presence of possible Fermionic-like excitations at the magnetic ground state. Although the bulk and film forms of Sr₃OsO₆ share the same lattice basis and electrically insulating state, the magnetism is entirely different between them.

Exotic magnetic behaviour and evidence of cluster glass and Griffiths like phase in Heusler alloys Fe2-xMnxCrAl (0 ≤ x ≤ 1)

We present a detailed study of structural, magnetic and thermodynamic properties of a series of Heusler alloys Fe_2-xMn_xCrAl (x = 0, 0.25, 0.5, 0.75 and 1). Structural investigation of this series is carried out using high resolution synchrotron X-ray diffraction. Results suggest that with increasing Mn concentration, the L2₁ structure of Fe₂CrAl is destabilized. The DC magnetization results show a decrement in paramagnetic (PM) to ferromagnetic (FM) phase transition temperature (T_C) with increasing Mn concentration. From the systematic analysis of magnetic memory effect, heat capacity, time dependent magnetization, and DC field dependent AC susceptibility studies it is observed that, Fe₂CrAl exhibits cluster glass(CG)-like transition approximately at 3.9 K (T_f2). The alloys, Fe_1.75Mn_0.25CrAl and Fe_1.5Mn_0.5CrAl exhibit double CG-like transitions near T_f1 ~ 22 K, T_f2 ~ 4.2 K and T_f1 ~ 30.4 K, T_f2 ~ 9.5 K respectively, however, in Fe_1.25Mn_0.75CrAl, a single CG-like transition is noted at T_f2 ~ 11.5 K below T_C. Interestingly, FeMnCrAl shows the absence of long ranged magnetic ordering and this alloy undergoes three CG-like transitions at ~22 K (T_f^*), 16.6 K (T_f1) and 11 K (T_f2). At high temperatures, a detailed analysis of temperature response of inverse DC susceptibility clearly reveals the observation of Griffiths phase (GP) above 300 K (T*) in Fe₂CrAl and this phase persists with Mn concentration with a decrement in T*.

Frustration and Glasslike Character in RIn1- xMn xO3 (R = Tb, Dy, Gd)

We bring together ac susceptibility and dc magnetization to uncover the rich magnetic field-temperature behavior of a series of rare earth indium oxides, RInO₃ (R = Tb, Dy, and Gd). The degree of frustration is much larger than expected, particularly in TbInO₃, and the ground states are glasslike with antiferromagnetic tendencies. The activation energy for spin reorientation is low. Chemical substitution with Mn³⁺ ions to form TbIn_{1- x}Mn _xO₃ ( x ≤ 0.01) relieves much of the frustration that characterizes the parent compound and slightly enhances the short-range antiferromagnetic order. The phase diagrams developed from this work reveal the rich competition between spin orders and provide an opportunity to compare the dynamics in the RInO₃ and Mn-substituted systems. These structure-property relations may be useful for understanding magnetism in other geometrically frustrated multiferroics.

Implications of bond disorder in a S=1 kagome lattice

Strong hydrogen bonds such as F···H···F offer new strategies to fabricate molecular architectures exhibiting novel structures and properties. Along these lines and, to potentially realize hydrogen-bond mediated superexchange interactions in a frustrated material, we synthesized [H₂F]₂[Ni₃F₆(Fpy)₁₂][SbF₆]₂ (Fpy = 3-fluoropyridine). It was found that positionally-disordered H₂F⁺ ions link neutral NiF₂(Fpy)₄ moieties into a kagome lattice with perfect 3-fold rotational symmetry. Detailed magnetic investigations combined with density-functional theory (DFT) revealed weak antiferromagnetic interactions (J ~ 0.4 K) and a large positive-D of 8.3 K with m_s = 0 lying below m_s = ±1. The observed weak magnetic coupling is attributed to bond-disorder of the H₂F⁺ ions which leads to disrupted Ni-F···H-F-H···F-Ni exchange pathways. Despite this result, we argue that networks such as this may be a way forward in designing tunable materials with varying degrees of frustration.

A rock-salt-type Li-based oxide, Li3Ni2RuO6, exhibiting a chaotic ferrimagnetism with cluster spin-glass dynamics and thermally frozen charge carriers

The area of research to discover new Li containing materials and to understand their physical properties has been of constant interest due to applications potential for rechargeable batteries. Here, we present the results of magnetic investigations on a Li compound, Li₃Ni₂RuO₆, which was believed to be a ferrimagnet below 80 K. While our neutron diffraction (ND) and isothermal magnetization (M) data support ferrimagnetism, more detailed magnetic studies establish that this ferrimagnetic phase exhibits some features similar to spin-glasses. In addition, we find another broad magnetic anomaly around 40–55 K in magnetic susceptibility (χ), attributable to cluster spin-glass phenomenon. Gradual dominance of cluster spin-glass dynamics with a decrease of temperature (T) and the apparent spread in freezing temperature suggest that the ferrimagnetism of this compound is a chaotic one. The absence of a unique freezing temperature for a crystalline material is interesting. In addition, pyroelectric current (I_pyro) data reveals a feature in the range 40–50 K, attributable to thermally stimulated depolarization current. We hope this finding motivates future work to explore whether there is any intriguing correlation of such a feature with cluster spin-glass dynamics. We attribute these magnetic and electric dipole anomalies to the crystallographic disorder, intrinsic to this compound.